The largemouth bass Micropterus salmoides Lacepede was given an intramuscular (IM) or intraperitoneal (IP) injection of Nocardia asteroides isolated from a diseased cultured largemouth bass, and typical granulomatous lesions similar to those of the natural infection were induced. The experimental fish showed 100% mortality 14 or 21 days after IP or IM injection of 8 mg of bacteria, and 28 days after IM injection of 0.8 mg of bacteria. The bacterium could be easily reisolated from the liver and kidney of dead fish. Many typical focal granulomata developed at the site of injection and in organs such as liver, spleen, kidney, eye, brain, pancreas, gill, stomach, intestine, and ovary. The result shows that the bacterium is clearly pathogenic to the largemouth bass.

The present paper describes a herpes-like virus, tentatively designated as EHVF (Eel Herpesvirus in Formosa), isolated from diseased Japanese eels (Anguilla japonica Temminck & Schlegel) in Taiwan.This virus had DNA as its nucleic acid, sensitive to ether and chloroform, and resistant to freezing and thawing treatments. Enveloped mature viral particle measured 200 nm in diameter with nucleocapsid of 120 nm.The virus successfully replicated in CCF, CCT, TO-2, BGK, DK-1 and EO-2 cell lines at temperatures from 15°C to 33°C (optimal temperature 24°C). When these cells were infected with EHVF, syncytium formations were observed in CCF, CCT, TO-2, BGK, EK-1 and EO-2 cell lines. However, HeLa, Vero, L, AF, AK, BF-2, CCG, CCO, CF, CHH-1, CHSE-214, EPC, FHM, FS, JF, LF, MH and RTG-2 cell lines were refractory to the multiplication of this virus.No mortalities were observed among eels injected with EHVF, but the virus was reisolated from the kidneys and livers of infected eels 60 days after injection. On the other hand, carp injected with EHVF showed 37% mortality during 60 days and the virus was reisolated from all the dead fish and 84% of the survivors.

A new epizootic occurred in red sea bream, Pagrus major, in Shikoku Island in 1990. Diseased fish swam inactively and showed severe anemia with 20-60% mortality. Typically enlarged cells characterized by bathophilic stainability were observed in the spleen, heart, kidney, liver and gill. From the morphology and localization of these cells, they are considered leucocytes. Hexagonal virions were found in the cytoplasm of these cells. Each virion consisted of a central electron-dense core (120 nm) and an electron translucent zone and measured 200-240 nm in diameter. Feulgen staining of the enlarged cells suggested DNA in the virus. These results suggest that the virus belongs to the family Iridoviridae. The virus slowly replicated and produced cytopathic effects of enlarged and rounded cells in RTG-2, CHSE-214, FHM, BF-2 and KRE-3 cells at 20-25°C. Intraperitoneal inoculation of the filtrate (450 nm) of spleen homogenate of the infected fish to red sea bream fingerlings induced similar pathological changes to those observed in spontaneously diseased fish.

A transmission experiment was carried out to evaluate the effects of Trichodina truttae infection on the growth and survival of juvenile chum salmon (Oncorhynchus keta) and the host's defense responses. The intensity of infection rose sharply to reach a peak of an average of 5, 730 (range 1, 770-12, 600) parasites per fish at week 3 post-infection, and then declined markedly to near zero by the end of the experiment (week 6). The parasite was site-specific to the skin of host fish and induced epidermal hyperplasia. Heavy infections caused intensive flashing among infected fish, which was accompanied by mass host mortalities with a cumulative loss of 56%. The growth and seawater adaptation of juveniles were, however, not significantly affected. In the course of infections, drastic changes occurred in the degree and histochemistry of the skin epidermal mucus secretion. The density of mucous cells containing acid mucopolysaccharides (alcian blue-positive, PAS-negative) decreased with increasing parasite intensity, while the parasite infections induced a marked increase of PAS-positive mucous cells, in turn, which may have caused a distinct reduction in parasite intensity. These results suggest that the increased production of PAS-positive mucous cells is an effective defense mechanism against trichodinid infections.

Experimental infection method for Paracolo disease (Edwacdsiella tarda infections) was studied to imitate histopathologically natural infections in the Japanese eel Anguilla japonica. Prior to the bacterial challenge, the intestine of eel was damaged by 0.1 or 0.05 ml of 30% hydrogen peroxide introduced through a silicon tube (1 mm in diameter) which was inserted 3 to 5 cm into the intestine from the anus. E. tarda was mixed with a sterilized eel diet and administered into the stomach by a cannula 18 hours after the hydrogen peroxide treatment. Bacterial dosesused were from 2.6&times;104 to 8.4&times;107 CFU/eel. Most fish that were challenged with doses from2.6&times;104to 2.6&times;106 CFU/fish formed abscess in either the liver or the kidney and died withi5-23days after the challenge. Histopathological changes of their kidneys or livers were principally identical to those observed in the natural infection. On the other hand, fish challenged with doses over 7.9&times;106 CFU/fish mostly underwent acute process without abscess formation and died within first 4 days after the challenge, though abscesses were fbrmein someme survived animals within 6-12 days